In process automation technology, analog or digital measurement signals representative of process variables are preferably generated using field mounted devices. The process variables may represent, for example, mass flow rate, tank contents level, pressure, temperature, etc., which are sensed with suitable sensors.
Such field mounted devices are commonly connected with one another and with associated process control computers via a suitable data transmission system, to which they send measurement signals via, e.g., a 4- to 20-mA current loop and/or via a digital data bus. For the data transmission systems, field bus systems, particularly serial systems, such as Profibus-PA, Foundation Fieldbus, CANbus, etc. and the corresponding communications protocols are used.
By means of the process control computers, the transmitted measurement signals are processed and visualized as corresponding measurement results, e.g., on monitors, and/or converted to control signals for actuators, such as solenoid valves, electric motors, etc.
Besides their primary function, namely to generate measurement signals, present-day field mounted devices have various additional functionalities which support an efficient and reliable control of the process to be monitored. These include such functions as self-checking by the field mounted device, storing measured values, generating control signals for actuators, etc. Because of this high functionality of the field mounted devices, process-controlling functions can be shifted increasingly to the field level, so that process control systems can be organized as correspondingly decentralized systems. These additional functionalities also relate, for example, to the start-up of the field mounted device and to its connection to the data transmission system.
These and further functions of field mounted devices are commonly implemented using field mounted device electronics, which comprise a microcomputer and suitable software that is part of the microcomputer. The software is programmed into a permanent storage, such as a PROM, or a nonvolatile, persistent storage, e.g., an EEPROM, of the microcomputer prior to or during the start-up of the field mounted device, and, for the operation of the field mounted device, can be loaded into a volatile storage, such as a RAM.
The processes monitored by the field mounted devices are subject to constant modification both with respect to the construction of the plants and with respect to the time sequences of individual process steps. Accordingly, the field mounted devices must be adapted to changing process conditions and further developed. This applies to the sensors, but particularly also to the implemented functions, such as the control of the sensor, the processing of the measurement signals, or the presentation of the measurement results, and to the communication with the data transmission system.
For such reconfigurations of the functions implemented in the field mounted device electronics, corresponding modifications of the stored software are necessary in programmable field mounted devices. Such reconfigurations are commonly made in situ via a control unit connected to the field mounted device, and may comprise, for example, changes to individual instrument parameters or the loading of complete processing programs.
One way of implementing the reconfigurations is to replace the storages containing software to be modified by storages containing the modified software, which can be done, for example, by rearranging individual storage elements or replacing the respective memory boards. This necessitates opening the housing of the field mounted device, so that the latter must be switched off.
Another disadvantage is that during such a reconfiguration, the affected plant sections also may have to be shut down.
If nonvolatile storage devices are used for storing the software, another possibility of reconfiguring the field mounted device electronics is to download the modified software from a mass storage, e.g., a diskette, a CD-ROM, or a magnetic tape, via a download bus into a volatile storage of the signal processing unit, e.g., into a RAM. The software to be modified can then be erased in the nonvolatile storage and be replaced by the software contained in the volatile storage.
For these reconfigurations, the housing of the field mounted device need not be opened; however, the field mounted device must be switched off-line, i.e., it must be put out of operation. This reduces the reconfiguration time considerably, but a shutdown of affected plant sections cannot be ruled out.
Another disadvantage of such a reconfiguration is that 5 the software to be modified is no longer loaded while the modified software has not been completely loaded yet. The longer this condition lasts, the higher the probability that external disturbances, such as power supply variations, will occur. Because of the relatively high power requirement for writing to nonvolatile storages and because of the generally small energy reserves in field mounted devices, particularly in fail-safe devices, this may result in the software to be loaded being erased from the volatile storage and thus being not available for the reconfiguration. Accordingly, the reconfiguration will terminate uncompleted. This erroneous condition can be overcome, for example, by resetting the microcomputer to preprogrammed default settings and reloading the software, but this prolongs the downtime of the field mounted device and possibly of the affected plant section.
In many cases, however, a reduction of error probability by increasing the available electric power using, for example, additional, energy-storing accumulator and/or capacitance networks conflicts with the demand for field mounted device electronics with minimum space and power requirements and with minimum circuit complexity. Furthermore, for many applications, particularly in hazardous areas, the storage capacity of energy storage circuits is limited by relevant standards to a maximum energy reserve.